1. Field of the Invention
The present invention relates to electrical circuit components. More particularly, the present invention relates to an overvoltage protection diode better capable of withstanding high energy electrical impulses.
2. Introduction to the Invention
Semiconductor diodes are usually defined as two-terminal, anode-cathode devices. Such diodes are most often constructed by adjacently forming in a wafer of semiconductor monocrystal p-type doped (positive electric charge carriers or “holes”) and n-type doped (negative electric charge carriers or electrons) regions or layers to realize a p-n junction. For example, a p-type epitaxial layer may be formed on an n-type wafer or substrate to form a p-n junction, or an n-doped epitaxial layer may be formed on a p-doped wafer or substrate to form an n-p junction.
Semiconductor diodes typically manifest much higher electrical conductivity in one direction of current flow (forward bias) than in the other direction (reverse bias). However, in an operating condition typically referred to as “breakdown”, for p-n diodes or p-n Zener diodes, when a certain reverse bias voltage level is reached (dependent upon diode fabrication details), reverse bias current flow occurs and rapidly transitions from a minute leakage current to a significant current flow.
Avalanche breakdown, which is the result of carrier “impact ionization”, is a process that occurs in a semiconductor's space charge region or depletion region under a sufficiently high electric field which is the result of the voltage difference between the two sides of the diode. At that high field the net electron/hole generation rate due to impact ionization exceeds a critical value, enabling the current to rise indefinitely due to a positive feedback mechanism. Zener breakdown, a different phenomenon, is the result of band-to-band quantum tunneling of charge carriers across the potential barrier created by the applied reverse bias. This phenomena occurs in heavily doped semiconductor material and at relatively low reverse voltage levels. For silicon devices, this voltage is on the order of three to six volts. There is no abrupt transition between Zener tunneling and impact ionization. As higher reverse bias voltages are applied across a p-n junction, more impact ionization current is encountered.
The magnitude of the reverse bias current flow may vary from a minute leakage current to a substantial current beginning at the reverse breakdown level. At the reverse breakdown level the voltage level across the p-n junction remains substantially fixed, thereby rendering such diodes effective as a voltage reference or as a voltage limiter or voltage regulator, while operating under reverse bias conditions. However, unless the current flow is effectively limited to some maximum level, current-resistance (I×R) heating caused during avalanche breakdown can rapidly and permanently degrade or destroy the semiconductor diode structure.
The critical field at which idealized breakdown occurs is frequently presented as a function of semiconductor doping levels per cubic centimeter (“/cm3”) and generally varies from 1014 to 1018 (five decades). Thus, it is known to create graphs which indicate an idealized breakdown voltage for a given (e.g. negative carrier or n+) doping level. In practical semiconductor diode devices, structural edge effects create high field concentrations, and it is very unlikely that an idealized breakdown voltage can be realized in a realizable semiconductor device. Yet, for many years workers in the field have tried to come up with ways to increase the breakdown voltage in practical device structures in an attempt to approach the ideal breakdown voltage.
One example of a prior effort to increase reverse avalanche breakdown voltage at the surface of a planar silicon diode is set forth in U.S. Pat. No. 3,391,287 to Kao et al., for “Guard Junctions for P-N Semiconductor Devices”. In this early patent one or more P-N junction “guard rings” were proposed to divide up the surface electric field into separated segments, so that surface breakdown was thereby forced to occur at a higher voltage level. Most of the problems that limit breakdown voltage are problems related to what happens at the surface of the silicon chip. So, as workers skilled in the art learned how to deal with these problems, they were able to develop devices capable of withstanding higher reverse breakdown voltages.
Conventional Zener diodes are essentially structures formed of planar layers that, under reverse bias electrical conditions below the avalanche breakdown voltage level, are analogous to oppositely facing plates of a capacitor. Depending upon how a particular semiconductor diode is fabricated, avalanche breakdown always takes place at a weakest point or area of the diode, i.e. at an area of highest electric field (measured as volts per micrometer). Since breakdown (and I×R heating) becomes concentrated at a weakest point or area of the diode junction, steps have been proposed in the prior art to limit or prevent breakdown at such weak points or areas.
While reverse avalanche breakdown is one problem confronting semiconductor diodes, it is not the only concern. When excessive power is dissipated in a semiconductor device, the resultant heating can damage, degrade, or destroy the device. Thus, another major problem is a semiconductor diode's survivability when it is confronted by a high energy condition, as predictably occurs in circuit protection devices and arrangements.
There have been a number of electrical and electronic devices that have been proposed in the prior art in efforts to solve electrical overstress and electrostatic discharge problems. Among these elements of the prior art are included ceramic capacitors, Zener diodes, transient voltage suppression (“TVS”) diodes and thyristors, multilayer varistors, gas-plasma ionization devices, and Schottky diodes, for example. TVS diodes typically add a regular diode in series with a Zener diode in an effort to lower net capacitance across the diode structure. These TVS diode/thyristor structures can take a variety of forms, such as four or five layer (PNPN) monolithic silicon devices having a self-gating circuit set to be triggered at a predetermined voltage level. Initial response to an overvoltage condition is a clamping or avalanche effect, quickly followed by a crowbar action. The thyristor remains latched in its low impedance state until the current falls to a level less than a holding current, whereupon the thyristor returns to a high-resistance off state. One example of a PNPN constant-voltage diode providing over-voltage protection is disclosed in U.S. Pat. No. 5,430,311 to Murakami et al. Needless to say, these multi-layer structures can become very complicated and relatively expensive to manufacture.
A multi-element protection arrangement is proposed, for example, by U.S. Pat. No. 4,901,183 to Lee. Therein, a series of electrical/electronic elements including fast acting fuses, metal oxide varistors, capacitive-inductive networks and silicon TVS devices are arranged in a staged fashion to provide a series of defenses to protect a load against an overvoltage energy pulse. Several embodiments of integrated ESD/overcurrent devices are described in U.S. Pat. No. 6,628,498 to Whitney et al. One of the devices described therein appears to be a Zener diode chip mounted on top of a surface mount polymeric positive temperature coefficient (PPTC) resistor device. While that patent mentions Zener diodes, it also proposes varistors or thyristors having characteristics useful for protecting against overvoltage conditions for different applications. Commonly assigned U.S. Pat. No. 6,518,731 to Thomas et al. includes embodiments wherein a Zener diode is mounted to, or in thermal contact with, a PPTC resistor device to provide thermal coupling between the two electrically interconnected circuit protection elements.
A hitherto unsolved need has remained for a simplified semiconductor diode structure that can withstand a higher energy pulse condition than can be withstood by conventional Zener diodes and reverse avalanche (impact ionization) diodes.